Electrical resistor with a parallel high voltage protective gap



Feb. 21, 1967 P. L. KIR'BY 3,305,694

' ELECTRICAL RESISTOR WITH A PARALLEL HIGH VOLTAGE PROTECTIVE GAP Filed April 30, 1965 INVENTOR PETER L/NLEV K/RBY United States Patent O 3,305,694 ELECTRICAL RESISTOR WITH A PARALLEL HIGH VOLTAGE PROTECTIVE GAP Peter Linley Kirby, Gosforth, England, assignor to Welwyn Electric Limited, Bedlington, England, a company of Great Britain and Northern Ireland Filed Apr. 30, 1965, Ser. No. 452,192 Claims priority, application Great Britain, May 12, 1964, 19,696/64 8 Claims. (Cl. 315-59) This invention relates to electrical resistors to high voltage electrical resistors.

It has been found that in resistors employed in high voltage circuits the resistance element, as well as the insulation material which protects the resistor surface is liable to break down due to very high voltage surges.

An electrical resistor which obviates this defect has been described in the specification of British Patent No. 918,666. In one form described in this prior specification, the electrical resistor has a hollow resistance carrier member upon which a resistive element is mounted or coated. The element is electrically connected between terminal caps provided at each end of the carrier memher. In order to reduce the likelihood of breakdown of the resistive element occurring a pair of electrodes is proparticularly vided inside the hollow carrier member, each electrode being connected to an end cap. The electrodes form a spark-gap in parallel with the resistive element thereby protecting the latter against the effect of excessively high voltage pulses.

The present invention is also closely concerned with obviating the chance of breakdown of the resistive element and it provides a spark-gap of improved form and having additional beneficial features.

According to the present invention there is provided an electrical resistor having a resistance carrier member upon which a resistive element is mounted or coated, and electrically connected between terminal members provided at each end of the carrier member, wherein the re sister is surrounded on the outer surface by an insulating layer, said insulating layer having some of its length covered by a conducting sleeve Which extends from one terminal member but terminates short of the other terminal member.

The latter terminal member preferably carries a metallic annular member Whose outer diameter is approximately equal to the outer diameter of the conducting sleeve. The edges of the conducting sleeve and annular member are preferably rounded. The insulating layer may be made of a silicone elastomer.

The metallic sleeve performs two functions. Firstly, it provides a shunt capacitance to the resistive path and thus reduces the voltage gradient along the resistor when transients with a steep wave front are applied. It thereby helps to distribute the Wave front in such a way as to reduce breakdown. Secondly, it acts as one electrode of a spark-gap, in a similar way to the spark-gap system described in British Patent No. 918,666.

In addition to the external spark-gap, an internal spark. gap may be provided, in which case it may be either constructed in accordance with the disclosure in the prior patent referred to above, or the electrodes may be hollow tubes. Where an internal spark gap is provided, the carrier member is hollow to allow the electrodes to be contained therein.

The present invention will now be described in greater detail by way of example with reference to the accompanying drawing, wherein the sole figure shows two high voltage electrical resistors connected in series. It will be seen that these are provided with an internal spark-gap having slightly different features from that disclosed in 3,305,694 Patented Feb. 21, 1967 British Patent No. 918,666, but it should be realised that this is an optional feature and does not in fact form part of the invention.

Each high voltage electrical resistor comprises a ceramic tube 1 upon which a resistance film 2 is deposited by any suitable known technique. The value of resistance of this film 2 is appropriately adjusted to the correct value. In one form this is done by cutting a helix along the greater part of the length of the film 2. The construction illustrated includes hollow electrodes 4 and 5 which are electrically connected to end terminals 3 to form a spark-gap within the hollow ceramic tube 1, the ends of the tubes extending out of the ends of the terminals 3. The electrodes 4 and 5 are in the form of metal tubes and each has an aperture 6 near the end remote from the terminals 3, so that if the end of the hollow electrodes 4 and 5 become sealed off by prolonged sparking, there still remains an alternative path for the access of gas to or from the inner part equalization of pressure tube 1.

A thick insulating layer 8 is applied over the resistive film 2. In a preferred form this thick layer 8 is composed of a silicone elastomer. A metallic sleeve 7 is provided over part of the insulating layer 8, one end being electrically connected to an end terminal 3. The other end terminal 3 of one resistor (the left-hand one) is provided with an annular metallic member 9 whose outer diameter is approximately the same as the outer diameter of the sleeve 7. The ends of the sleeve 7 and member 9, are rounded or turned over as shown at 10, in the manner preferredfor the construction of electrical sparkgaps.

The sleeve 7 performs an additional function. It provides a shunt capacitance to the resistive path and thus reduces the voltage ating the wave front if voltage transients are applied to the resistor. Experience has shown that when such transients occur, and especially when several resistors are connected in a series chain the distributed electrical parameters can act in a manner of a transmission line which can lead to the rapid propagation of a voltage wave-front with the development of excessive voltages along the. resistive element, resulting in breakdown of the insulation at the input end of the resistor. The provision of the sleeve 7 helps to distribute the Wave front in such a way as to reduce breakdown from this cause.

Yet another advantage of the external sleeve 7 is that it serves the purpose of an anti-corona device when the particular resistor which it surrounds is operating at a high potential With respect to earth potential. In this case it serves to ensure that each resistor is surrounded by electrostatic fields no greater than its normal maximum operating field. If the resistor is maintained at a high potential with respect to earth the danger of corona discharge from those parts of the assembly having a sharp profile (such as the edge of the helical groove, or the end terminals 3) is now practically eliminated. The smooth external contour of the sleeve 7 greatly reduces the likelihood of corona discharge at any given absolute voltage level.

The length of the sleeve 7 is determined by the distance between the sleeve and the annular member 9 which is designed to provide a specified breakdown voltage at a level greater than the normal maximum working voltage of the resistor and less than the level which is known to cause damage to the element. This distance is designated 11. As illustrated it is further the normal practice to use resistors of this type in series chains. When this occurs, the relevant spark-gap is provided between the leading edge of the sleeve 7 on one resistor and the trailing inside and outside the ceramic of the ceramic tube 1, allowing 3 edge of the sleeve ion the adjacentresistor, and in this case there is no annular member 9 applied to the opposite end of the resistor to that which is connected to the sleeve 1. In this instance the critical breakdown distance is determined once again by the length of the sleeve.

What I claim and desire to secure by Letters Patent is:

1. An electrical resist-or comprising a resistance carrier member; a resistive element coated. over said carrier member; terminal members provided at each end of the carrier member, the resistive element being electrically connected to said terminals; an insulating layer surrounding the outer surface of the resistive element, and a conducting sleeve provided over some of said insulating layer, said sleeve extending from one terminal member but terminating short of the other terminal member.

2. An electrical resistor comprising a resistance carrier member; a resistive element mounted over said carrier member; terminal members provided at each end of the the carrier member, the resistive element being electrically connected to said terminals; an insulating layer surrounding the outer surface of the resistive element; a conducting sleeve provided over said insulating layer and extending from one terminal member to a short distance from the other, and a metallic annular member provided on the other terminal member, the diameter of said annular member being approximately equal to the outer diameter of the conducting sleeve.

3. An electrical resistor comprising a hollow resistance carrier member; a resistive element mounted over said hollow carrier member; terminal members located at each end of the carrier member, the resistive element being electrically connected to said terminals; an insulating layer surrounding the outer surface of the resistive element; a hollow electrode connected to each terminal member, the electrodes extending towards each other inside the hollow resistance carrier member for defining a spark gap; a conducting sleeve provided over said insulating layer and extending from one terminal member to a short distance from the other, and a metallic annular member located on the other terminal member, the diameter of said annular member being approximately equal to the outer diameter of the conducting sleeve.

4. An electrical resistor according to claim 3, wherein each hollow electrode has an aperture near the end remote from the terminal members.

5. An electrical resistor comprising a hollow resistance carrier member; a resistive element coated over said hollow carrier member; terminal members located at each end of the carrier member, the resistive element being electrically connected to said terminals; a layer of silicone elastomer surrounding the outer surface of the resistive element; a solid electrode connected to each terminal member, the electrodes extending towards each other inside the hollow resistance carrier member for defining a spark-gap; and an electrically conducting sleeve provided over some of said silicone elastomer layer, said sleeve extending from one terminal member but terminating short of the other terminal member.

6. An electrical resistor comprising a hollow resistance carrier member; a resistive element coated over said hollow carrier member; terminal members provided at each end of the carrier member, the resistive element being electrically connected to said terminals; an insulating layer surrounding the outer surface of the resistive element, the resistance value being adjusted to the correct value by cutting a helix along the greater part of the length of the resistive element; a hollow electrode connected to each terminal member, the electrodes extending towards each other inside the hollow resistance carrier member; a conducting sleeve provided over said insulating layer and extending from one terminal member to a short distance from the other; and a metallic annular member provided on the other terminal member, the diameter of said annular member being approximately equal to the outer diameter of the conducting sleeve, the distance between the end of the conducting sleeve and the metallic annular terminal member determined .so as to provide a specified breakdown voltage at a level greater than the normal maximum working voltage of the resistor and less than the level which is known to cause damage to the resistive element.

7. An electrical resistor according to claim 6, wherein the edges of the conducting sleeve and annular terminal member are rounded.

8. Two or more electrical resistors according to claim 2, connected end to end to form a series chain, the metallic annular member being omitted from all the resistors except the first.

No references cited.

JAMES W. LAWRENCE, Primary Examiner. C. R. CAMPBELL, Assistant Examiner. I 

6. AN ELECTRICAL RESISTOR COMPRISING A HOLLOW RESISTANCE CARRIER MEMBER; A RESISTIVE ELEMENT COATED OVER SAID HOLLOW CARRIER MEMBER; TERMINAL MEMBERS PROVIDED AT EACH END OF THE CARRIER MEMBER, THE RESISTIVE ELEMENT BEING ELECTRICALLY CONNECTED TO SAID TERMINALS; AN INSULATING LAYER SURROUNDING THE OUTER SURFACE OF THE RESISTIVE ELEMENT, THE RESISTANCE VALUE BEING ADJUSTED TO THE CORRECT VALUE BY CUTTING A HELIX ALONG THE GREATER PART OF THE LENGTH OF THE RESISTIVE ELEMENT; A HOLLOW ELECTRODE CONNECTED TO EACH TERMINAL MEMBER, THE ELECTRODES EXTENDING TOWARDS EACH OTHER INSIDE THE HOLLOW RESISTANCE CARRIER MEMBER; A CONDUCTING SLEEVE PROVIDED OVER SAID INSULAT- 